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Question

Comments

we have to calculate the total number of nop instructions which need to be inserted without using operand forwarding ??

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Can u plz post the image clearly?

Actually though they have told not to use operand forwarding, but they have described  NOP phase same as that.

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The instructions given are-

ADD R3,R2,R4    R3<-R2+R4

SUB R6,R4,R3    R6<-R4-R3

ADD R7,R5,R3   R7<-R5+R3

SUB R1,R7,R4   R1<-R7-R4

The number of NOP instructions which need to be inserted without operand forwarding are=??

My answer is 6 but madeeasy answer is 4.

Answer 1

Inpipelining NOP phase is the phase where no productive work is done.

Our aim is to do some useful work in NOP phase. Actually it is the phase, where some cycles are wasted.

In this phase, pipeline work same with data dependency.

Though , no operand forwarding is used in this phase , but due to data dependency, it produce some stalls and also some NOP phase.

Number of stalls are 8 among them 4 are NOP phase.

Comments

i am not able to understand the difference between nop and stalls?? can u plzz explain. According to me we insert nop in stall cycles. so how 4 NOP and 4 stalls?? Its so confusing. I am lost

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The number of bubble inside pipelining are called stalls.(see anyreference u will get it).

I think The extra bubble we have to add due to data dependency, are in NOP phase.(I have not got any reference yet, but searching for it)

Answer 2

Here, Case 2 is applicable because it is explicitly mentioned in the question that WB Stage is divided into 2 parts- 1st Part is Write, 2nd Part is Read